Quartz glass is distinguished by a low coefficient of thermal expansion, by optical transparency over a wide wavelength range, and by high chemical and thermal resistance. The term “quartz glass” refers to doped or undoped quartz glass with a high silicic acid content, namely, a SiO2 content of at least 85%
To achieve special properties, quartz glass is doped with other substances, such as titanium, aluminum, boron or germanium. These substances are added in very small amounts and act as foreign atoms in the glass structure without forming a second phase in addition to the matrix.
A quartz glass which contains an additional component is here designated as “black glass,” wherein a second phase of carbon, silicon, silicon carbide, silicon nitride, titanium nitride or titanium carbide is intercalated in a matrix of quartz glass and consolidated into a composite material. The fine-particle regions of the intercalated phase (e.g., Si phase or carbon phase) act as optical defects and have the effect that the composite material has a black or gray visual appearance at room temperature. On the other hand, the defects also have impacts on the heat absorption or heat emission of the composite material on the whole. Thermal radiation between 2 μm and 8 μm, i.e., in the wavelength range of the infrared radiation, is strongly absorbed. The high absorption and emission capacity for thermal radiation reduces the reflection thereof on the surfaces of the composite material. Local, non-reproducible heating up by reflected thermal radiation is thereby prevented and a uniform temperature distribution is achieved in the environment of the composite material. The high degree of emission of the composite material makes the component particularly suitable for use in thermal treatments where a reproducible and homogeneous temperature distribution is of importance.
Frequently, there is a need to connect quartz glass elements to one another, for example for the manufacture of quartz glass components having a complex shape. Commonly, this connection is provided by welding the components to one another. For example, EP 1 042 241 B1 describes a method for the butt-like welding of quartz glass tubes. Welding comprises melting the surfaces to be connected to one another and pressing the softened surfaces against one another, which may easily lead to undesired plastic deformation in the area of the welding zone. Though it is possible to remove deformations of this kind by extensive after-treatment, some dimensional deviations usually remain.
Further difficulties arise during the welding of black glass. During heating, the temperature is possibly limited to values below the melting point of the intercalated phase (Si phase: 1410° C.). In the case of intercalated carbon, the limiting factor is that carbon detaches from the free surface due to strong heating or it oxidizes or burns off due to the high process temperatures; this may lead to the formation of bubbles. Moreover, due to the above-mentioned high emission degree of the black glass, most of the heat that has been introduced by a welding torch is emitted again, so that the material is shining in a brightly glowing manner and rapidly cools down again, as compared with transparent or opaque quartz glass. A bonding or shaping of black glass is therefore not possible by way of hot working. It is known from EP 2 048 121 A1 that a component of black glass is provided on all sides of the surface with a layer of transparent quartz glass. Thanks to the layer, it is possible to process the component of black glass in hot processes, specifically to weld it to another quartz glass component. The transparent outer layer is generated by subjecting a porous SiO2 soot body to a gas phase reaction with organic silicon compounds, followed by vitrification under vacuum. The carbon component of the organic Si compound cannot diffuse out of the core region of the soot body during vitrification, resulting in a component of black glass with a carbon phase in the core region (carbon content 30 wt. ppm to 50,000 wt. ppm) and with a carbon-free (less than 30 wt. ppm carbon) transparent outer layer. The outer layer has a layer thickness in the range of one millimeter to 10 millimeters, the layer thickness depending on the negative pressure set during vitrification of the component. The setting of the layer thickness and the adjustment of the carbon amount in the core region of the component turn out to be complex, so that the method according to EP 2 048 121 A1 must be considered to be complicated on the whole. Moreover, this will yield only a component of black glass that is provided on all sides with a transparent outer layer; the selective formation of only one connecting surface is not possible.
A component of black glass with a transparent outer layer is also disclosed in US 2014/0072811 A1. A plate of black glass is here brought into contact with an SiO2-containing slurry mass after sintering at 1500° C. so that the plate is provided on all sides with a thin slurry layer. This is followed by temperature treatments at 1000° C. and 1600° C., respectively, resulting in a densely sintered plate of black glass with a transparent, bubble-free outer layer having a thickness of 0.1 mm. Since an already sintered plate is used, the adhesion of the slurry mass to the surface is relatively poor, so that one obtains only a thin slurry layer and, as a result thereof, also only a 0.1 ram thin, transparent outer layer. Such a thin outer layer is not sufficient for subsequent bonding by welding because, as has been explained above, the heat of a welding torch immediately penetrates the outer layer and acts on the black glass positioned thereunder, it is there emitted again, and the component thus cools down all in all too rapidly to allow bonding with another material having a high silicic acid content.
Furthermore, DE 10 2004 054 392 A1 discloses a method for connecting components consisting of a material with a high silicic acid content, in which method a pourable or pasty slurry mass which contains amorphous SiO2 particles is applied to individual connecting surfaces of densely sintered quartz glass components. Immediately thereafter, the connecting surfaces are fixed with respect to or onto each other. The bonding mass that is more or less enclosed between the two connecting surfaces is then dried. A correspondingly slow drying process yields a dried layer without cracks that is suited for connecting relatively small plates. This method is no longer suited for large-area connections with a correspondingly high pressing pressure already due to the dead weight of the quartz glass parts to be connected, or it would require long and uneconomic drying times to remove the dispersing agent of the bonding mass from the bonding site without any defects.